This project is being submitted in two separate grant proposals to facilitate administration of funds. See accompanying proposal with the same title by C. Richard Taylor, Alexander Agassiz Professor of Zoology, Museum of Comparative Zoology, Harvard University. This proposal has two main objectives: 1) To determine the manner in which muscles and populations of fibers within muscles are recruited during locomotion. 2) To investigate the contribution of skeletal muscle to total resting metabolism. This is the third year of a five year project. The specific goals for this third year remain the same as those proposed in the initial proposal. They are to determine: 1) the relationship between cross-sectional area of muscle fibers showing glycogen loss and the force exerted by the muscle during normal locomotion; 2) how muscles and muscle fibers within muscles are recruited when active muscles shorten and perform positive work and they are stretched to perform negative work; 3) the role of gaits (walk, trot or run, gallop) as a means for maximizing muscular efficiency and minimizing energetic cost of exercise; 4) to correlate the patterns of electrical activity of muscles with length changes and force generation in individual muscles during normal locomotion; 5) fiber composition of major locomotory muscles in a wide diversity of animals, and the contribution of skeletal muscles to total metabolism at rest and during exercise. Progress has been made in each of these areas during the first two years of the proposal. We are adding a sixth goal for the coming year: To determine whether the temperature of a muscle affects the order of recruitment of muscle fibers during normal locomotion.